Infrared absorption spectroscopy (IR) is a spectral analysis method based on the characteristic absorption of infrared radiation by a substance. After molecules absorb infrared radiation, transitions in vibrational energy levels and rotational energy levels occur, so infrared spectroscopy is also called molecular vibration-rotation spectroscopy.
Group Vibration and Infrared Spectrum Region
Various groups have their own characteristic infrared absorption frequencies. According to the relationship between spectral characteristics and molecular structure, infrared spectra can be divided into two major regions: group frequency region and fingerprint region.
1. Group frequency region (4000~1350 cm-1)
The functional group region is the characteristic vibration frequency region of chemical bonds and groups. Its absorption spectrum reflects the vibration of characteristic groups in molecules, and the characteristic absorption peak can be used as a basis for identifying groups. The base frequency region can be divided into the following regions.
1) X-H stretching vibration region (4000~2500 cm-1)
X represents atoms such as C, O, N, and S. O-H stretching vibration is at 3700~3100 cm-1, and alcohols, phenols, organic acids, and water molecules have strong absorption in this region. The N-H stretching vibration is between 3500 and 3300 cm-1. Primary and secondary amides as well as primary and secondary amines have absorption bands in this region. The N-H band overlaps with the O-H band, but the N-H absorption peak is sharp. The O-H band often has a lower frequency and a wider band due to the presence of hydrogen bonds. The C-H stretching vibration of saturated hydrocarbons is below 3000 cm-1, and the C-H stretching vibration of unsaturated hydrocarbons (including olefins, alkynes, and aromatics) is above 3000 cm-1.
2) Triple bond and cumulative double bond region (2500~1900 cm-1)
This region has fewer infrared bands, mainly including asymmetric stretching vibrations of -C≡C and -C≡N triple bonds and asymmetric stretching vibrations of cumulative double bonds such as -C=C=C- and -C=C=O.
3) Double bond stretching vibration region (1900~1200 cm-1)
This region mainly includes stretching vibrations of C=O, C=C, C=N, N=O, and skeleton vibrations of benzene rings, as well as the overtones of aromatic compounds. The stretching vibration of the C=C bond (olefin bond) appears at 1680~1620 cm-1. Its intensity is generally weak and depends on the symmetry of the olefin. Strong symmetry may even be non-infrared active. The stretching vibration of the C=O bond appears in the range of 1850~1600 cm-1. It is often the strongest absorption peak in the infrared spectrum, and the possibility of interference from other absorption peaks in this area is relatively small. Therefore, the C=O stretching vibration absorption peak is the main basis for judging the presence of carbonyl groups. In addition, the position of the C=O absorption peak is also closely related to the adjacent groups, which is of great value for judging the type of carbonyl compounds. The C=C stretching vibration (i.e., the skeleton vibration of the aromatic ring) of monocyclic aromatic hydrocarbons has four absorption peaks in the range of 1620~1450 cm-1, of which the two absorption peaks in the regions of 1520~1480 cm-1 and 1620~1590 cm-1 are important signs for judging the presence of aromatic rings.
Benzene derivatives have overtone peaks of C-H out-of-plane bending vibration in the region of 2000~1667 cm-1. Although the intensity is very weak, the shape and number of the absorption peaks are related to the substitution type of the aromatic ring. The substitution type of the benzene ring can be determined by using the absorption peak in this region and the C-H out-of-plane bending vibration absorption peak of the benzene ring in the 900~600 cm-1 region.
4) X-H bending vibration region (1650~1350 cm-1)
This region includes C-H and N-H bending vibrations. Methyl has a very characteristic bending vibration absorption peak at 1380~1370 cm-1. The position of this absorption peak is rarely affected by the substituent and has less interference. It can be used as a basis for judging whether a methyl group exists. When there are two methyl groups on a carbon atom, the bending vibrations of the two methyl groups couple with each other, causing the absorption peak near 1370 cm-1 to split, resulting in two absorption peaks.
2. Fingerprint region (1350~650 cm-1)
The absorption peak in the fingerprint region is caused by the stretching vibration of C-C, C-O, C-X single bonds and the bending vibration of most groups in the molecular skeleton. Since the strength of various single bonds is roughly the same, the interaction between adjacent single bonds makes the absorption spectrum in this region very complex and very sensitive to slight changes in structure. As long as there are slight differences in chemical structure (such as homologues, isomers and spatial conformation, etc.), there will be obvious reflection in the fingerprint region. Just like human fingerprints, the fingerprints of two people cannot be exactly the same, and the infrared spectra of two compounds are also different in the fingerprint region. Therefore, the fingerprint region is very useful for identifying compounds.
In the fingerprint region, there are also several characteristic absorption frequencies of groups. For example, the absorption peak of the =C-H out-of-plane bending vibration of olefins depends on the substitution of double bonds. The absorption peak of the trans configuration is at 990~970 cm-1, while the cis configuration appears near 690 cm-1. This can be used as a judgment of cis-trans isomers. The absorption band of the C-H out-of-plane bending vibration of the benzene ring appears at 900~650 cm-1. It is very characteristic for determining the substitution type of the benzene ring. Although the absorption peak position of the C-O stretching vibration varies greatly (1300~1000 cm-1), it is not difficult to determine the existence of the C-O bond because the absorption intensity is also very large.
| Stretching Vibrations | Bending Vibrations | |||||
| Functional Class | Range (cm-1) | Intensity | Assignment | Range (cm-1) | Intensity | Assignment |
| Alkanes | 2850-3000 | strong | CH3, CH2 & CH | 1350-1470 | medium | CH2 & CH3 deformation |
| 2 or 3 bands | 1370-1390 | medium | CH3 deformation | |||
| 720-725 | weak | CH2 rocking | ||||
| Alkenes | 3020-3100 | medium | =C-H & =CH2 (usually sharp) | 880-995 | strong | =C-H & =CH2 |
| 1630-1680 | variable | C=C (symmetry reduces intensity) | 780-850 | medium | (out-of-plane bending) | |
| 675-730 | medium | cis-RCH=CHR | ||||
| 1900-2000 | strong | C=C asymmetric stretch | ||||
| Alkynes | 3300 | strongong | C-H (usually sharp) | 600-700 | strong | C-H deformation |
| 2100-2250 | variable | C≡C (symmetry reduces intensity) | ||||
| Arenes | 3030 | variable | C-H (may be several bands) | 690-900 | strong-medium | C-H bending & |
| 1600 & 1500 | medium-weak | C=C (in ring) (2 bands) | ring puckering | |||
| (3 if conjugated) | ||||||
| Alcohols & Phenols | 3580-3650 | variable | O-H (free), usually sharp | 1330-1430 | medium | O-H bending (in-plane) |
| 3200-3550 | strong | O-H (H-bonded), usually broad | 650-770 | variable-weak | O-H bend (out-of-plane) | |
| 970-1250 | strong | C-O | ||||
| Amines | 3400-3500 (dil. soln.) | weak | N-H (1°-amines), 2 bands | 1550-1650 | medium-strong | NH2 scissoring (1°-amines) |
| 3300-3400 (dil. soln.) | weak | N-H (2°-amines) | 660-900 | variable | NH2 & N-H wagging | |
| 1000-1250 | medium | C-N | (shifts on H-bonding) | |||
| Aldehydes & Ketones | 2690-2840(2 bands) | medium | C-H (aldehyde C-H) | |||
| 1720-1740 | strong | C=O (saturated aldehyde) | 1350-1360 | strong | α-CH3 bending | |
| 1710-1720 | strong | C=O (saturated ketone) | 1400-1450 | strong | α-CH2 bending | |
| 1100 | medium | C-C-C bending | ||||
| 1690 | strong | aryl ketone | ||||
| 1675 | strong | α, β-unsaturation | ||||
| 1745 | strong | cyclopentanone | ||||
| 1780 | strong | cyclobutanone | ||||
| Carboxylic Acids & Derivatives | 2500-3300 (acids) overlap C-H | strong | O-H (very broad) | 1395-1440 | medium | C-O-H bending |
| 1705-1720 (acids) | strong | C=O (H-bonded) | ||||
| 1210-1320 (acids) | medium-strong | O-C (sometimes 2-peaks) | ||||
| 1785-1815 ( acyl halides) | strong | C=O | ||||
| 1750 & 1820 (anhydrides) | strong | C=O (2-bands) | ||||
| 1040-1100 | strong | O-C | ||||
| 1735-1750 (esters) | strong | C=O | ||||
| 1000-1300 | strong | O-C (2-bands) | 1590-1650 | medium | N-H (1¡-amide) II band | |
| 1630-1695(amides) | strong | C=O (amide I band) | 1500-1560 | medium | N-H (2¡-amide) II band | |
| Nitriles | 2240-2260 | medium | C≡N (sharp) | |||
| Isocyanates, Isothiocyanates, Diimides, Azides & Ketenes | 2100-2270 | medium | -N=C=O, -N=C=S | |||
| -N=C=N-, -N3, C=C=O | ||||||
Special Attention in Infrared Spectroscopy in Structural and Component Identification and Analysis
The characteristics of infrared spectroscopy are fast analysis speed; a wide range of samples that can be measured, including gas, liquid, solid and polymer elastomers; sample analysis is a non-destructive technology with a small amount of sample, generally at the milligram level, and microanalysis technology can reach the microgram level. As a quantitative analysis method, infrared spectroscopy is cumbersome to prepare samples, and the peaks selected for quantitative measurement are affected by a variety of interference factors, resulting in large errors, generally between 5% and 10%. Therefore, infrared spectroscopy is most widely used for functional group structure analysis of molecules and identification analysis of different samples. Matters that require special attention in analysis are as follows:
1. Pay attention to the source of the sample and related physical and chemical properties data
Before analysis, try to understand the source, purpose, elemental composition, and possible structural type, physical and chemical properties of the sample as much as possible. If you have sufficient knowledge of the source and purpose of the sample, you can usually predict the type and composition range of the sample structure.
2. Pay attention to the IR measurement experimental technology of the sample
Analyze whether the sample preparation amount and sample concentration are appropriate, whether the maximum absorption peak has a flat-top saturation suppression peak; adjust the optical path balance and whether there is interference from water vapor and CO2 gas; whether the transparency of the tablet is uniform during the tablet method, whether the spectral baseline has upward drift and baseline correction, etc.
3. Pay attention to whether the purity of the sample can provide a sufficiently reliable infrared spectrum.
The spectral purity required by infrared spectroscopy for sample determination is not high, and it can provide effective information if it can reach 90%-95%. This also shows that it is not sensitive to the detection of small components in the sample with a content of only a few percent, especially trace components. For example, sometimes two samples with completely different colors visually give the same infrared image; for many homologues with different relative molecular masses but similar structures, similar or identical infrared images can be given.
4. Identify "false peaks" and "ghost peaks" that are easy to appear in IR images
The "false peaks" that appear in infrared spectroscopy analysis are mainly caused by instruments and analysis methods, such as the CO2 peak in the light path at 2235 cm-1 due to light path imbalance; the H2O interference fringe peak at 2500-1600 cm-1; the water peak adsorbed in the potassium bromide tablet at 3400 cm-1, etc. Sometimes there are some peaks that are unrelated to the sample structure and experimental technology. Their characteristics are: irregular size and shape, sometimes appearing and sometimes not, and irregular positions, commonly known as ghost peaks. Some ghost peaks are caused by impurities introduced during the sample separation process. For example, the broad and strong Si＝O bond peak at ~1100 cm-1 may be due to the use of high vacuum silicone grease as a sealant or silica gel as an adsorbent to separate the sample in the synthesis reaction; it may even appear after distilled water is used as a solvent and evaporated. The use of petroleum ether solvents to bring in 1380, 1460 and 2900 The alkane absorption peaks near cm-1; the plasticizer phthalates that appear in the evaporation residues of most eluting organic solvents are at 2900, 1720, 1280 cm-1 absorption peak. The way to identify these occasional ghost peaks is to use the same experimental conditions to perform a blank control experiment to confirm them.
5. Pay attention to the peak position, shape, intensity and related peaks
In the analysis, the first thing to pay attention to is the position of the absorption peak, that is, the wave value of the peak; secondly, pay attention to the shape of the peak, such as sharp peaks, broad peaks, the presence of shoulders and peak splitting; thirdly, pay attention to the peak intensity, whether it is strong, medium or weak; fourthly, pay attention to the various related peaks that may appear in different positions for the same functional group.
6. Pay special attention to the comprehensive analysis of spectra in the structural analysis of unknown substances
It is sometimes difficult to give a definite conclusion by relying solely on a single infrared spectral structure interpretation and comparison analysis. For this reason, it should be combined with a comprehensive analysis of organic mass spectrometry and nuclear magnetic resonance spectroscopy to provide consistent structural information, so as to give a more reliable conclusion.
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